Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique in which a user transmits on many orthogonal frequencies (or sub-carriers). The orthogonal sub-carriers are individually modulated and separated in frequency such that they do not interfere with one another. This provides high spectral efficiency and resistance to multipath effects. An orthogonal frequency division multiple access (OFDMA) system allows some sub-carriers to be assigned to different users, rather than to a single user. Today, OFDM and OFDMA technology are used in both wireline transmission systems, such as asymmetric digital subscriber line (ADSL), and wireless transmission systems, such as IEEE-802.11a/g (i.e., WiFi) and IEEE-802.16 (e.g., WiMAX). This technology is also used for wireless digital audio and video broadcasting.
Code division multiple access (CDMA) is a single carrier transmission technique in which multiple users transmit in the same frequency band using different codes. CDMA technology is also widely used in both wireline and wireless applications, such as IS-95 and IS-2000 cellular networks.
Conventional power amplifiers for amplifying multi-carrier signals are relatively expensive parts of a communication system. A key design parameter for OFDM, OFDMA and CDMA power amplifiers is the requirement to minimize the generation of adjacent channel power (ACP) noise. ACP noise results from signal distortion caused by operation of power amplifier components in non-linear regions of the input-output characteristic such as when the power amplifier enters saturation. This distortion produces undesired spectral components in adjacent transmission channels.
Power amplifiers are more linear at lower input signal levels. Large input signals tend to drive a power amplifier into saturation. Thus, weaker signals experience less distortion and have lower ACP noise. However, a sudden large signal peak still results in distortion and ACP noise. This is especially problematic in systems that have large peak-to-average power ratios (i.e., high crest factors), such as OFDM, OFDMA and CDMA systems. To avoid this, power amplifiers often operate in “back-off” mode (i.e., reduced input signal) in order to accommodate large signal peaks. However, operating in back-off mode requires the use of devices with higher power ratings which adds to system design, development and manufacturing costs. Furthermore, it may be inefficient and may generate excessive heat.
Therefore, there is a need for improved OFDM, OFDMA and CDMA transmission system that minimize amplifier peak-to-average power ratio (PAPR) or crest factor without suffering performance degradation.